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SubjectRe: [PATCH v10 1/7] sched/fair: Provide u64 read for 32-bits arch helper
On Tue, Jun 07, 2022 at 01:32:48PM +0100, Vincent Donnefort wrote:

> From: Vincent Donnefort <vincent.donnefort@arm.com>
>
> Introducing macro helpers u64_u32_{store,load}() to factorize lockless
> accesses to u64 variables for 32-bits architectures.
>
> Users are for now cfs_rq.min_vruntime and sched_avg.last_update_time. To
> accommodate the later where the copy lies outside of the structure
> (cfs_rq.last_udpate_time_copy instead of sched_avg.last_update_time_copy),
> use the _copy() version of those helpers.
>
> Those new helpers encapsulate smp_rmb() and smp_wmb() synchronization and
> therefore, have a small penalty for 32-bits machines in set_task_rq_fair()
> and init_cfs_rq().
>
> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 77b2048a9326..05614d9b919c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -612,11 +612,8 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq)
> }
>
> /* ensure we never gain time by being placed backwards. */
> - cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
> -#ifndef CONFIG_64BIT
> - smp_wmb();
> - cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> -#endif
> + u64_u32_store(cfs_rq->min_vruntime,
> + max_vruntime(cfs_rq->min_vruntime, vruntime));
> }
>
> static inline bool __entity_less(struct rb_node *a, const struct rb_node *b)
> @@ -3313,6 +3310,11 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags)
> }
>
> #ifdef CONFIG_SMP
> +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> +{
> + return u64_u32_load_copy(cfs_rq->avg.last_update_time,
> + cfs_rq->last_update_time_copy);
> +}
> #ifdef CONFIG_FAIR_GROUP_SCHED
> /*
> * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list
> @@ -3423,27 +3425,9 @@ void set_task_rq_fair(struct sched_entity *se,
> if (!(se->avg.last_update_time && prev))
> return;
>
> -#ifndef CONFIG_64BIT
> - {
> - u64 p_last_update_time_copy;
> - u64 n_last_update_time_copy;
> -
> - do {
> - p_last_update_time_copy = prev->load_last_update_time_copy;
> - n_last_update_time_copy = next->load_last_update_time_copy;
> -
> - smp_rmb();
> -
> - p_last_update_time = prev->avg.last_update_time;
> - n_last_update_time = next->avg.last_update_time;
> + p_last_update_time = cfs_rq_last_update_time(prev);
> + n_last_update_time = cfs_rq_last_update_time(next);
>
> - } while (p_last_update_time != p_last_update_time_copy ||
> - n_last_update_time != n_last_update_time_copy);
> - }
> -#else
> - p_last_update_time = prev->avg.last_update_time;
> - n_last_update_time = next->avg.last_update_time;
> -#endif
> __update_load_avg_blocked_se(p_last_update_time, se);
> se->avg.last_update_time = n_last_update_time;
> }
> @@ -3796,12 +3780,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> }
>
> decayed |= __update_load_avg_cfs_rq(now, cfs_rq);
> -
> -#ifndef CONFIG_64BIT
> - smp_wmb();
> - cfs_rq->load_last_update_time_copy = sa->last_update_time;
> -#endif
> -
> + u64_u32_store_copy(sa->last_update_time,
> + cfs_rq->last_update_time_copy,
> + sa->last_update_time);
> return decayed;
> }
>
> @@ -3933,27 +3914,6 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> }
> }
>
> -#ifndef CONFIG_64BIT
> -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> -{
> - u64 last_update_time_copy;
> - u64 last_update_time;
> -
> - do {
> - last_update_time_copy = cfs_rq->load_last_update_time_copy;
> - smp_rmb();
> - last_update_time = cfs_rq->avg.last_update_time;
> - } while (last_update_time != last_update_time_copy);
> -
> - return last_update_time;
> -}
> -#else
> -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> -{
> - return cfs_rq->avg.last_update_time;
> -}
> -#endif
> -
> /*
> * Synchronize entity load avg of dequeued entity without locking
> * the previous rq.
> @@ -6960,21 +6920,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> if (READ_ONCE(p->__state) == TASK_WAKING) {
> struct sched_entity *se = &p->se;
> struct cfs_rq *cfs_rq = cfs_rq_of(se);
> - u64 min_vruntime;
> -
> -#ifndef CONFIG_64BIT
> - u64 min_vruntime_copy;
> -
> - do {
> - min_vruntime_copy = cfs_rq->min_vruntime_copy;
> - smp_rmb();
> - min_vruntime = cfs_rq->min_vruntime;
> - } while (min_vruntime != min_vruntime_copy);
> -#else
> - min_vruntime = cfs_rq->min_vruntime;
> -#endif
>
> - se->vruntime -= min_vruntime;
> + se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> }
>
> if (p->on_rq == TASK_ON_RQ_MIGRATING) {
> @@ -11422,10 +11369,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
> void init_cfs_rq(struct cfs_rq *cfs_rq)
> {
> cfs_rq->tasks_timeline = RB_ROOT_CACHED;
> - cfs_rq->min_vruntime = (u64)(-(1LL << 20));
> -#ifndef CONFIG_64BIT
> - cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> -#endif
> + u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20)));
> #ifdef CONFIG_SMP
> raw_spin_lock_init(&cfs_rq->removed.lock);
> #endif
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 1f97f357aacd..bf4a0ec98678 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -520,6 +520,45 @@ struct cfs_bandwidth { };
>
> #endif /* CONFIG_CGROUP_SCHED */
>
> +/*
> + * u64_u32_load/u64_u32_store
> + *
> + * Use a copy of a u64 value to protect against data race. This is only
> + * applicable for 32-bits architectures.
> + */
> +#ifdef CONFIG_64BIT
> +# define u64_u32_load_copy(var, copy) var
> +# define u64_u32_store_copy(var, copy, val) (var = val)
> +#else
> +# define u64_u32_load_copy(var, copy) \
> +({ \
> + u64 __val, __val_copy; \
> + do { \
> + __val_copy = copy; \
> + /* \
> + * paired with u64_u32_store, ordering access \

s/u64_u32_store/u64_u32_store_copy()/

> + * to var and copy. \
> + */ \
> + smp_rmb(); \
> + __val = var; \
> + } while (__val != __val_copy); \
> + __val; \
> +})
> +# define u64_u32_store_copy(var, copy, val) \
> +do { \
> + typeof(val) __val = (val); \
> + var = __val; \
> + /* \
> + * paired with u64_u32_load, ordering access to var and \

s/u64_u32_load/u64_u32_load_copy()/

> + * copy. \
> + */ \
> + smp_wmb(); \
> + copy = __val; \
> +} while (0)

Try again here from me.
The semantics of this macro is different from the original code.
To be consistent with the original code, the 'copy = __val;' should
be changed to 'copy = var'. Why not original code here.

They are different in that the var can be changed currently on another
CPU and copy can be different from __val.

Documentation/memory-barriers.txt(line 1351, 5.19.0-rc1-rt1):
In section:
(*) What are memory barriers?
- Multicopy atomicity.
The example there can be referenced here I think.

And I feel that here the smp_wmb() should be changed to smp_mb(). One user
of this macro have the load/write write. Another user have write write
if I am not wrong.

Also,

'Memory Barriers: a Hardware View for Software Hackers' (July 23, 2010)
Section: 6.3 Example 2, I just scrolled to the code in table 3 and check
that the semantics there is almost the same with the semantics here.

I do not have confidence to convince anything just after some days when I
read it.

According to those two examples, if the small penalty is small, this
u64_u32_store_copy() macro should be changed like this:

# define u64_u32_store_copy(var, copy, val) \
do { \
typeof(val) __val = (val); \
var = __val; \
/* \
* paired with u64_u32_load_copy(), ordering access to var and \
* copy. \
*/ \
smp_mb(); \
copy = var; \
} while (0)

Not sure and this is the continuation to my reply to this patch in v4.

Thanks,
Tao
> +#endif
> +# define u64_u32_load(var) u64_u32_load_copy(var, var##_copy)
> +# define u64_u32_store(var, val) u64_u32_store_copy(var, var##_copy, val)
> +
> /* CFS-related fields in a runqueue */
> struct cfs_rq {
> struct load_weight load;
> @@ -560,7 +599,7 @@ struct cfs_rq {
> */
> struct sched_avg avg;
> #ifndef CONFIG_64BIT
> - u64 load_last_update_time_copy;
> + u64 last_update_time_copy;
> #endif
> struct {
> raw_spinlock_t lock ____cacheline_aligned;
> --
> 2.36.1.255.ge46751e96f-goog

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